The revisions to the criminal procedure law were unveiled at the National People's Congress on Thursday, and will be passed by what is essentially a rubber stamp parliament. Lawyers have welcomed other elements offering additional protection to suspects. Reformers have long battled for such measures – which include improving legal access, increasing protection for juveniles and outlawing evidence gathered through torture – although many are sceptical about how they will be observed.

"The real issue is not what the laws say, but how they are enforced," Pu Zhiqiang, a Beijing lawyer who has taken on sensitive cases such as those involving dissidents, said.

"The pattern is that the Communist party can play by rules when there aren't special circumstances – but whenever there are special circumstances, it doesn't have to play by them."

The new law states that police have the right to hold suspects or defendants at places other than their home or an official detention centre if they are "involved in crimes concerning state security, terrorism or especially serious corruption and notification of where they are residing could obstruct investigations".

Experts said that that police would not have to disclose where they were holding suspects, although they believed in theory suspects would have a right to legal access.

State security crimes include subversion and other vaguely-worded charges often used against dissidents.

But the clause that most alarmed campaigners – by stating that, in such cases, police did not have to notify families within 24 hours if it could impede the investigation – has now been removed.

"It makes a world of difference whether you notify people or not," Nicholas Bequelin, of Human Rights Watch, said. "People inside and outside the system have prevented this from being introduced. It was not an easy battle, and it has to be noticed."

But Rosenzweig said the law was still "extremely problematic". He said: "Detention of this kind was originally intended to be relatively lenient, but I have heard from people held this way that they would rather have been in a jail.

"It is still six months where the police do not have to really justify anything they do to you."

Wang Songlian, of the Chinese Human Rights Defenders Network, added: "Detention centres are not pleasant, but they at least have minimum safeguards and standards.

"[With this] there is a high risk of mistreatment and torture over a very long time."

Chen Guangzhong, a professor at the Chinese University of Political Science and Law in Beijing, who criticised earlier drafts – including the secrecy clause – said the revised law marked a necessary curtailment of police powers.

"This change is significant," he said. "It means China is making further effort in improving human rights, democracy and the rule of law. In the past, judicial authorities had too much power."